Kelvin Controls | Humidity Control

The Importance of Humidity Control within the Built Environment

The Effects of Dry Air on Our Body

People can tell whether an atmosphere is hot or cold the moment we experience it but air that has a low or high humidity is not obvious to us. Our senses are not so refined as to “feel” the moisture content of the air we breathe or which touches our skin and bodies. However, the effects of air’s humidity on our “Wellbeing” is significant and can be damaging when extremes are experienced over a long period.

The laws of physics define that dry air will draw moisture from any available source it comes into contact with, including our bodies. Water can evaporate from our eyes, skin, hair, nails and any other exposed surface. This dehydration can cause initial discomfort, such as sore eyes or contact lens irritation, but can also lead to more serious symptoms, such as dry, itchy skin and dermatitis.

Even more serious is the effect it has on our respiratory and immune systems. When we breathe air below 40%RH, the mucous membranes in our nose, throat and bronchi dry. These elements play a vital role in our body’s defence against airborne pollutants, such as viruses and bacteria. These moist membranes capture airborne particles before they enter our lungs. Tiny hairs, called cilia, transport these pollutants to our throats where they are either coughed out or swallowed and destroyed. This process is referred to as mucociliary clearance. When our mucous membranes dry out, this natural defence mechanism is inhibited, leaving us more susceptible to airborne infection from viruses and bacteria.


Physiological and subjective responses to low relative humidity

Sunwoo Y et al, 2006

J Physiol Anthropol. 2006 Jan;25(1):7-14

Nasal mucociliary transport in healthy subjects is slower when breathing dry air

Salah B et al, 1998

Eur Respir J. 1988 Oct;1(9):852-5

On the mucus flow rate in human nose

Ewert G, 1965

Acta Otolaryngol Suppl. 1965;200:SUPPL200:1-62

The dichotomy of relative humidity on indoor air quality

Wolkoff P, Kjargaard SK, 2007

Environ Int. 2007 Aug;33(6):850-7. Epub 2007 May 17

The Effects of Low Humidity in the Built Environment

Low air humidity below 40%RH acts as a conduit for viruses and airborne bacteria to disperse and travel around the build environment. This occurs due to a combination of an increased virus survival time at lower humidity and the enhanced suspension of the infectious particle in dry air.

Whenever an infected person breathes, talks, coughs or sneezes they release aerosolized droplets that contain elements including - saliva, mucus, salts, germs and viruses into the air. Large droplets fall to the ground or settle on surfaces but droplets less than 4 microns in size have been shown to remain airborne for hours.

Within the built environment, room air expelled droplets lose more than 90% of their moisture content rapidly by evaporation after they are released. At a humidity level of above 40%RH, elements like salts and proteins are still dissolved in the droplet but become highly concentrated. They attack viruses and bacteria, rendering them inactive, reducing the risk of secondary infection. However, below the critical level of 40%RH, further evaporative moisture losses result in these elements crystallising out of solution. The internal environment of the airborne droplets changes from aggressive to protective towards the transported germs inside. This enables airborne viruses and bacteria to remain infectious for longer.

The lower humidity also causes more droplets to evaporate down to a size capable of remaining airborne. So dry air has the double effect of creating a greater quantity of airborne droplets and prolonging the infectivity of the germs they carry, significantly increasing the potential risk of secondary infection.


High humidity leads to loss of infectious influenza virus from simulated coughs

Noti JD et al, 2013

PLoS One. 2013; 8 (2):e57485

Criteria for human exposure to humidity in occupied buildings

Sterling EM et al, 1985

ASHRAE Transactions, Vol. 91, Part 1, CH85-13 No. 1, 1985

Dynamics of airborne influenza A viruses indoors and dependence on humidity

Yang W, Marr LC

PLoS ONE, 1 June 2011 | Volume 6 | Issue 6 | e21481

Influenza and humidity - why a bit more damp may be good for you!

Jane A. Metz, Adam Finn

Journal of Infection (2015) 71, S54eS58

Further Reading – Guidance on Humidity Levels within the Built Environment

HSE: Work with display screen equipment Health and Safety (Display Screen Equipment) Regulations 1992 as amended by the Health and Safety (Miscellaneous Amendments) Regulations 2002

BSRIA: Applications Guide AG10.94.1 Efficient Humidification in Buildings, March 1995

CIBSE: Guide A, Environmental design, 8th edition, March 2015

CIBSE: Knowledge Series, KS19: Humidification, Oct 2012

ASHRAE: Position Document on Airborne Infectious Diseases 2014

Kelvin Control Engineers Ltd: Psychrometric Chart

Should you wish more information on the above or simply be looking for advice on what we can offer please contact Kelvin Controls, where we can discuss all options and if necessary arrange a site visit to give appropriate advice tailored to your budget.